It should work™. Check if the kwin_drm_devices environment variable is applied correctly in your kwin_wayland process, this oneliner should help you (run it inside your plasma wayland session)
cat /proc/$(pidof kwin_wayland)/environ | tr '\0' '\n' | grep -i kwin
To make sure your variable is appled correctly (before kwin_wayland starts), you can export it in the global profile. Create a file /etc/profile.d/kde.sh with just this content
export KWIN_DRM_DEVICES=/dev/dri/card1:/dev/dri/card0
and reboot.
If the variable is applied correctly, it could happen that simple drm device messed up the DRI nodes order, then you can show us what is the output of
ls -l /dev/dri/by-path/
if card0 is mapped to nvidia, and card1 to intel, then you would need to switch that in KWIN_DRM_DEVICES variable, too.